docs: def _add_suffix(file_path, suffix, sep='_', ext=None):
      """Adds suffix to a file name seperated by an underscore and returns file path."""
      # Split file_path on last '.' to separate file_name and file_extension
      split = os.path.basename(file_path).rsplit('.', 1)
  
      # Use original file_extension if None is given
      ext = split[1] if not ext else ext.strip('.')
  
      # Rebuild new file_path with suffix
      return os.path.join(os.path.dirname(file_path), split[0] + sep + suffix + '.' + ext)